Carbon nanotubes (CNTs) are used as cold cathode electron sources for field emission applications. These applications include displays (see U.S. Pat. No. RE38,223; and C. G. Lee et al., “FEDs with CNT on large area applications,” Proceedings of the 9th International Display Workshops, p. 1021, Dec. 4–6, 2002, Hiroshima, Japan), x-ray tubes (see U.S. Pat. No. RE38,223; and G. Z. Yue et al. “Generation of continuous and pulsed diagnostic imaging x-ray radiation using a carbon-nanotube-based field-emission cathode,” Applied Physics Letters, Vol. 81, pp. 344–357), microwave devices (Chris Bower et al., “A micromachined vacuum triode using a carbon nanotube cold cathode,” IEEE Transactions on Electron Devices, Vol. 49, No. 8, p. 1478, August 2002), satellite thrusters and neutralizers and other applications requiring a source of electrons. In some cases, the carbon nanotube film or layer is grown on the substrate using various chemical vapor deposition (CVD) techniques known in the state of the art. These films do not generally require an activation process and are used as grown. However, there are many advantages to fabricating the carbon nanotube (CNT) cathode by dispensing or printing the CNT layer. See U.S. Patent Publication No. US-2003-0092207-A1, incorporated by reference herein. These inks or pastes are deposited onto the substrate by screen printing, dispensing, ink-jet printing, spraying, painting or other such means.
These processes have several advantages over growing the CNT material onto the substrate using CVD techniques. The dispensing and printing processes do not require process temperatures much above 450° C.–500° C.; the CNT fabrication process (generally at 600° C. and above) is separated from the dispensing process. There are many vendors that supply many different sizes and characteristics of CNT, so optimal material for field emission applications can be identified. Furthermore, printing and dispensing processes are demonstrated to be low-cost and can be scaled to large area in high-volume manufacturing environments. These processes are also capable of making CNT films having superior field emission properties (low threshold fields, high current capability, etc.).
A problem with these printing or dispensing approaches is that they often require an activation process. These activation processes include laser blasting or laser beam activation (see Junko Yotani et al., “CNT-FED for Character Displays,” Society for Information Display 2004 International Symposium Digest of Technical Papers, Vol. 35, Book II, p. 828–831, May 26–27, 2004; and also S. Nakata et al., “Fabrication of CNT Electron Source by Simple Stacking for Obtaining Uniform Emission Distributions,” Society for Information Display 2004 International Symposium Digest of Technical Papers, Vol. 35, Book II, p. 928–931, May 26–27, 2004; and W. Rochanachirapar et al., “Effect of laser irradiation on CNT-cathodes in different atmospheres,” Proceedings of the 10th International Display Workshops, p. 1207–1210, Dec. 3–5, 2003, Fukuoka, Japan), exposure to Ar ion plasma (see Yasunori Kanazawa et al., “Improvement in electron emission from carbon nanotube cathodes after Ar plasma treatment,” Journal of Vacuum Science and Technology B, Vol. 22, p. 1342–1344, 3 Jun. 2004), air jet and surface scratching or rubbing (see Kwang-Bok Kim et al., “Efficient electron emissions from printed carbon nanotubes by surface treatments,” Journal of Vacuum Science and Technology B, Vol. 22, p. 1331–1334, 3 Jun. 2004), or tape activation (see U.S. Publication No. US-2003-0092207-A1; and Yu-Yang Chang, Jhy-Rong Sheu, and Cheng-Chung Lee, “Method of improving field emission efficiency for fabricating carbon nanotube field emitters,” U.S. Patent Application Publication 2002/0104603 A1; and Daniel T. Colbert et al., “Method for growing continuous carbon fiber and compositions thereof,” U.S. Patent Application Publication 2002/0109086 A1).
Tape activation generally requires an adhesive tape and a roller or laminator. After the CNT paste is dispensed onto the substrate and processed at high temperature (e.g., 300° C.–500° C.) to eliminate organic binders and solvents and cure the paste, the tape is applied to the substrate such that the adhesive side of the tape is fixed to the top of the substrate and the CNT printed pattern. The tape is fixed by lamination or rolling or other means. The tape is then removed from the substrate, and during this process the top layer of the CNT paste is removed, exposing a fresh layer of CNT (not exposed to curing environment). The CNT density may also be altered and some of the CNTs may be aligned vertically to the substrate (parallel to the substrate normal). A problem with tape activation is that:                1) adhesive material may be left on the substrate,        2) it may be difficult to scale to large area and high volume,        3) structures on the substrate may be damaged or changed,        4) the adhesive layer on the tape may not be uniform over large area, and        5) it may not result in uniform emission properties because of differences in applied pressure during lamination across the surface of the substrate, differences in adhesion and structures already on the substrate that prevent the tape from being applied to certain areas of the CNT paste.        
An example of the last point is shown in FIG. 1. The CNT film is printed onto a conducting line on an insulating substrate. In this case, only one pixel is shown, but the same is true for substrates having many pixels. Insulating layers are also printed on the substrate to act as spacer layers on which to attach a grid after the activation process is completed. It is difficult to apply the tape near the edges of the grid spacer layers since the tape and lamination rollers are not compliant enough to form around these structures. This results in incomplete activation of the CNT pixel and non-uniform emission properties of the printed CNT layer in the pixel area.
Laser beam activation may also have the same uniformity issues near other structures. It may also damage the CNT because of the high heat from the laser beam. It may also not be scalable to high volume manufacturing of large area displays.
Mechanical or surface scratching also has the same problems. Mechanical brushes would not be able to reach to the bottom of a well as shown in FIG. 1 to activate a CNT layer printed at the bottom of the well. Mechanical brushes may also adversely change the direction of alignment of the CNT fibers.
What is needed in the art is an activation process that can be lower cost, can be scaled to high volume and improves the field emission uniformity in the cathode.